Hard frame selection

Frames: The basics of screen printing The typical hard frame is welded from a single metal strip. It is known for its stability. The method of fixing the screen is to glue it to the metal frame. When selecting a hard frame, consider the following factors: 1. The size of the frame and the image; 2. Image resolution; 3. The tension of the target screen; 4. Off-grid clearance; 5. Screen type; 6. Screen angle; 7. Frame weight. The frame size is a significant factor because it not only fits the size of the printing machine, but also fits the size of the image to be printed. Of course, in order to facilitate the accurate transmission of the image, a screen frame that is appropriately larger than the image size may be selected, that is, a part of the screen outside the image is in a blank state. The larger the area of ​​the blank screen, the less pressure is applied to the screen during printing, which is advantageous for maintaining the accuracy of the image. The resolution of the image determines the number of lines used. The number of lines affects the size of the pull force. A good mesh frame should be able to withstand the tension of the screen, including the extra tension generated in order to overcome the off-gauge height when printing. It must also be able to withstand the extra tension created by the stretching to compensate for the slackening of the mesh. The force generated during printing or stretching is mainly related to the type of screen material. Sometimes it is necessary to adjust the angle of the screen relative to the frame, but this happens rarely. However, when it is necessary to make adjustments, it should be noted that if it does not match the tension on the sides of the frame, it may cause deformation of the frame. From the processing point of view, the weight of the frame is also important when transferring screens in the workshop. Moreover, the heavy weight of the frame on the machine, especially the roller press, may reduce the printing speed, register accuracy and increase the wear of machine parts. Different hard frame characteristics can be obtained by various methods. Figure 4 shows some of the characteristics of the frame. Except for solid metal CD printing screen frames and similar screen frames used in the electronics industry, most of them are made of hollow metal strips pressed from various materials. Aluminum is the most commonly used material. Steel can also be used, it is cheaper than aluminum, but it is heavy, and if it is not stainless steel, it is also prone to corrosion. Figure 4: Hard Frame Features Description These examples show the properties of the hollow metal strip used to make the frame. It comes in many types, including squares, rectangles and trapezoids (the trapezoid is designed for easy cleaning). When ordering rigid screen frames, it must be ensured that the thickness of the metal strip satisfies the requirements of the screen tension. Also note that if too much metal strip material is removed when the frame is reused, the performance of the mesh wall (where it is used to stick the mesh) may be attenuated. Optimizing the properties of a rigid frame No matter how good the new frame is, it cannot always be in good condition. From the budget point of view, they should belong to consumer goods in the printing, but people often regard them as important equipment, which is why at the end of the life of the network frame, it is also the primary reason for trying to continue using it. The shop floor manager restricts the scrapping of scrap boxes on the shop floor because they see the box as an important consumer product. We should adopt the principle of avoiding the occurrence of problems, rather than the principle of solving the problems after they occur. Frames should be discarded before they exceed acceptable flatness and stability. But in many workshops, printers use a curved deformed frame to perform rough work, leaving the frame as a fine work. Under certain conditions, the definition of the "best" frame becomes subjective, which makes it difficult to manage the inventory of frames. One way to solve this problem is to mark its characteristics each time the net frame is used, record its elasticity after printing, and see if it still has the necessary elasticity. Based on the recorded value, you can determine how many times the mesh frame can be stretched repeatedly in the "good" state. Set a limit under the maximum number of repeats. Outside this limit, the frame must be discarded or replaced. Of course, reducing the number of reuse of network frames (increasing the number of new network frames used) will increase the cost of screen production. However, this sacrifice will bring more profits in other areas. The choice of the first time to use the frame is very important, because the frame is quite flat before stretching. Stretching the screen to the surface of an uneven net frame can cause loose areas of voids or tiny stretches. Usually, the printer uses a multi-layered emulsion to compensate uneven surfaces, but this is not always the best solution. Because it takes a lot of frame preparation time. The new net frame is often subject to surface pre-roughening to improve the adhesion of the net. When a heavily stretched net frame is used, the wire mesh is removed and the residue of the glue is also removed, providing a flat and rough surface. However, removing too much metal may weaken the frame's strength. Each time you rub the frame, you reduce its strength and even remove a little bit of the surface of the frame, then the accumulation of multiple repetitive stretches will be sufficient to reduce the tensile strength of the frame, causing bending and distortion. Deformation. A simple way to determine how much material is removed is to weigh the frame before and after sanding. For example, a 2.4x1.6 in (6040 mm) rectangle, 0.12 in (3 mm) thick and 0.24 in. (6 mm) wide metal strip, if rubbed to reduce the weight by 10%, the metal strip surface thickness is reduced by 46%. Grinders are often used during the frame preparation phase, but they have the possibility of removing too much of the frame material. And unless the control is very careful during grinding, a serrated mesh surface will be obtained, and the mesh frame of the serrated surface may scratch the mesh during use. A better tool for heavy-grip rubbing is a rotating belt frosted, which does not rub off material excessively. When the frame is used repeatedly, it is usually cleaned with a volatile solvent, which will remove some of the particles and deposits, which may otherwise affect the adhesion when it is stretched. In addition, properly prepare and maintain the frame. It is important to understand the effect of temperature on the frame. Elastic screens have large temperature changes, whether they are stored, transported, exposed, or printed. During this process, they may expand, contract, and bend. For example, the temperature of a platemaking shop may be 68 degrees Fahrenheit (20 degrees Celsius), and the vacuum frame temperature may exceed 130 degrees Fahrenheit (54 degrees Celsius). According to experience, a change of 24 degrees Fahrenheit (13 degrees Celsius) will cause a 4N/cm elastic change of the frame, and the required change range is ±1 N/cm. In order to avoid this happening, it should be ensured that the frame is prepared and used at the same temperature.